1. Field of the Invention
The present invention relates to a fluid machine used, for example, in a supercharger of a vehicle.
2. Description of the Related Art
A rotor is disclosed in Japanese Laid-open Utility Model 63-198401, and a screw type compressor is disclosed in Japanese Laid-open Patent 4-311694.
The rotor is a male screw rotor for a screw type compressor, and includes a rotor shaft fixed to a rotor main body. The screw type compressor has mutually engaged male and female screw rotors, and each rotor includes a rotor shaft fixed to a rotor main body.
The conventional male screw compressor rotor is thick in tooth thickness and solid. Such rotors have a large moment of inertia. When a screw type compressor with a rotor having a large moment of inertia is used in a supercharger, the engine drive energy loss is large, the response in acceleration is poor, and a clutch for connecting and disconnecting the engine must be larger in size. Accordingly, in such rotors, hollow parts are formed in protrusions of the rotor main body to reduce the weight, and hence decrease the moment of inertia.
When processing a rotor main body without a hollow space by casting, the rotor shaft is fixed by the insert when casting the rotor main body. When processing a rotor main body by cutting, a shaft hole is drilled in a round bar of a larger diameter than a completed rotor. The rotor shaft is fixed in this shaft hole, and the rotor protrusion is machined on the outer circumference of the round bar. These solid rotors have little in misalignment between the center of rotation of the rotor main body and the rotor shaft.
In the case of a hollow rotor, on the other hand, a shaft hole is machine in the rotor main body, and the rotor shaft is fixed in this shaft hole. When machining the shaft hole, a proper position for chucking the rotor main body is necessary. However, in a hollow rotor, there is no chucking position other than the hollow space of the protrusion or the outer circumference of the rotor main body. Actually, there is no chucking place in the hollow space of the rotor. In the case of the rotor, the inner circumference of the hollow space is a curved surface, and an accurate plate for chucking is not prepared. If a shaft hole is machined by chucking such curved inner circumference, misalignment of rotor main body and rotor shaft is likely to occur.
When the outer circumference of the hollow rotor is chucked, the hollow protrusion is deformed by the clamping force of chucking, and this deformation causes misalignment of the shaft hole. Moreover, if the hollow rotor at the time of chucking has not yet had its protrusion formed, aside from the misalignment due to deformation, misalignment of the shaft hole due to forming of the protrusion is added, and a greater misalignment occurs.
In the case of casting the rotor main body and forming the hollow space by a core, the misalignment of the core and shaft hole deviates the balance of rotation substantially, and it needs a huge cost for correction of the balance.